This invention relates to an optical-head for performing reproduction, recording, or erasing of optical information on an optical-medium such as optical disk, and relates more particularly to an optical-head for controlling the quantity of optical beam emitted from an optical-source.
An optical-head employing a beam-splitter as a means to monitor the quantity of optical-beam emitted from an optical source had been conventionally used. A typical example of this is now explained by referring FIG. 4 wherein the optical-beam 102 emitted from the semiconductor laser 101 is introduced into the collimator lens 103, and is transformed into a parallel optical beam which is then introduced into the beam-splitter 104. A part of the optical-beam branched by beam-splitter 104 is introduced into the detector 105 monitoring its light quantity.
The output signal of detector 105 is fed back to the semiconductor laser 101 through the laser control circuit 112 in order to control the quantity of light emitted from semiconductor laser 101 at a proper level.
On the other hand, optical-beam 107 passed through beam-splitter 104 is introduced into the objective-lens 106 after it is reflected by the reflection mirror 108 at nearly right angle, and the optical-beam passed through objective-lens 105 is focused on the information medium 109 by using objective-lens 106.
The focused optical-beam is then reflected by information medium 109, and the reflected optical-beam is introduced into beam-splitter 104 taking a reversed path. The optical-beam reflected by beam-splitter 104 is introduced into the detecting lens 110 and is inputted into signal detector 111 yielding a focus servo-signal, tracking servo-signal and an information signal.
However, since the beam-splitter consists of at least two triangular prisms adhered together, on which anti-reflection layers are deposited, the dimension of beam-splitter is inevitably bulky and the cost is high.